RE: YS 170 DZ CDI
Hi Sleeping,
the line running from the pump to the throttle body (it's not really a carburetter), is a high pressure supply to the fuel metering arrangement in the throttle body.
The line running from the throttle body up along the intake tube to the black check valve on the head carries that metered fuel up to the check valve where it's injected into the motor. No fuel (apart from a small amount bypassed by the sub needle valve) goes into the motor crankcase via the throttle body. It all gets squirted in at the intake valve as the valve opens, as below the black check valve there's a small needle that is pointed at the intake valve.
When I prime the motor for the first start of the day, I fill the tank, draw fuel to the pump then connect the line to the pump. Open the throttle flat out and rock the engine back and forth and you'll see the fuel discharge from the pump in pulses, travel to the throttle body, then up to the check valve. Once it's at the check valve, a couple more rocks, close the throttle, power up the ignition and it good to go....